1. The Field of the Invention
The present invention down-the-hole tools and to down-the-hole drilling mechanisms in particular.
2. The Relevant Technology
While many different drilling processes are used for a variety of purposes, in most drilling process a drill head applies axial forces (feed pressure) and rotational forces to drive a drill bit into a formation. More specifically, a bit is often attached to a drill string, which is a series of connected drill rods that are coupled to the drill head. The drill rods are assembled section by section as the drill head moves and drives the drill string deeper into the desired sub-surface formation. One type of drilling process, rotary drilling, involves positioning a rotary cutting bit at the end of the drill string. The rotary cutting bit often includes (tungsten carbide or optimally, synthetic diamonds, TSD or PCD cutters) that are distributed across the face of the rotary cutting bit.
The rotary cutting bit is then rotated and ploughed into the formation under significant feed pressure. The velocity of each cutting element depends on the angular rotational rate of the bit and the radial distance of the element from the center of the bit. On a solid drill bit, the angular rotational rate will be the same for the entire bit. Accordingly, at any given speed those cutting elements nearer the outer edge will be travelling faster than those near the center of the bit.
As the drill string rotates the rotary cutting bit, the drill string can distort due to whirling or helical buckling. Helical buckling can cause the drill string to contact the walls of the hole, thereby generating frictional forces between the drill string and the walls. Accordingly, the rotational rate of the drill string can be controlled to control the frictional forces between the drill string and the walls of the hole.
In broken or unconsolidated formations that are difficult to drill, the hole walls can be sensitive to lateral pressure from the drill string and therefore speed is often limited to avoid whirling and helical buckling of the drill string which can damage the hole. This can in turn prevent the drill string from moving the cutting elements near the center of rotation at a sufficient speed to provide adequate penetration. Further, the torsional and frictional loads described above can cause helical buckling of the drill string, which in turn can damage the walls of the hole. If the hole becomes lost due to damage to the walls, the hole needs to be re-drilled, which can be extremely expensive.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one exemplary technology area where some embodiments described herein may be practiced.